This application is based on and claims the benefit of European Patent Application No. 00401068.2 filed Apr. 17, 2000, which is incorporated by reference herein.
The invention concerns a process for producing conducting wires coated with cross-linked polyethylene, in which process a material made of polyethylene is coated with a fluid cross-linking agent, the coated material is melted in an extruder and extruded onto the electrical conducting wire, and the extruded layer is cross-linked by heating to a temperature above the decomposition temperature of the cross-linking agent.
A process for the production of electrical conductors coated with cross-linked polyethylene is known from published German patent application DE 14 94 046 in which a polyethylene granulate is coated with a molten cross-linking agent at normal pressure, the granulate is melted in an extruder and homogenized, and the melt is applied to the electrical conductor. Molten dicumyl peroxide is used as a cross-linking agent. The cross-linking of the insulating layer occurs in a continuous vulcanization plant (cross-linking plant) at a temperature above the decomposition temperature of the cross-linking agent and at an elevated pressure.
Medium high-voltage or high-voltage cables with an insulating layer made of polyethylene have, in some circumstances, defects in the form of cavities, impurities, and surface defects which could lead to so-called electrical tree formation. In the presence of moisture and electrical load, so-called water trees could occur, which lead to destruction of the insulating layer. Attempts have been made to prevent these disadvantages by using so-called triple extrusion, i.e., the internal conducting layer, insulating layer, and external conducting layer are produced simultaneously through coextrusion by means of a triple extruder head. Impurities and cavities in the insulating layer and/or between the layers can hereby be minimized.
Another method performs a dry cross-linking instead of the steam cross-linking, with the heat transferred to the cable insulation by means of radiation.
Neither solution has been fully satisfactory. Optimization of the insulation material has also been attempted. Thus, the addition of so-called water tree inhibitors, which are derived from polymers, such as, e.g., organopolysiloxane, polyethylene glycol, epoxide resins, polypropylene, polystyrene, and polyvinyl alcohol, to the polyethylene during its production has been described. The desired effects and the influence on water tree growth vary greatly. In many cases, the additives have a deleterious effect on the homogeneity and the aging stability of the polyethylene, as well as on the electrical properties of the insulation material.
It is therefore an object of the present invention to providing a process for the production of medium high-voltage and/or high-voltage cables which is significantly more economical than the previously known processes and provides cable which has a higher resistance to the formation of water trees and therefore a longer service life.
This object is achieved according to the present invention by using in the coating material a mixture of granulate, meal, or powder made of a polyethylene homopolymer and a polyethylene copolymer together with a cross-linking agent and stabilizer, with the portion of copolymer in the coating on the cable lying between 1 and 8 weight percent.
The essential advantage of the invention can be seen in that by the use of a mixture of polyethylene homopolymer and polyethylene copolymer with a limited amount of copolymer, the resistance of the cable insulation to the formation of water trees can be significantly increased. Polyethylene homopolymer offered at a reasonable price by numerous polymer producers as a sort of staple item, largely for processing into polyethylene films, can be used. Therefore, expensive xe2x80x9cspecialtiesxe2x80x9d of the polymer producers can be dispensed with.
Further features and advantages of the invention will be apparent from the description and claims below.
The invention will be described in more detail with reference to several exemplary embodiments.